1. Field of the Invention
The present invention relates to a motion picture camera including an automatic focusing mechanism having a high degree of precision.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 3,836,772, motion picture cameras of the type having an automatic focusing device have, until now, been arranged to provide an in-focus signal as a result of a computation based on the outputs of detection arrays, one of which represents a fixed image formed in a fixed visual field within a distance measuring module, and the other a movable image formed by scanning of a movable optical scanning member. The illumination of each of the fixed and movable images provided by the arrays depends n the brightness of the object to be photographed. With low object brightness, therefore, the distance measuring accuracy decreases due to a drop in the output of the detecting arrays.
In a motion picture camera, the minimum allowable diameter of a scattering circle on a focal plane is smaller than that for a still picture camera, while the depth of focus for a motion picture camera is shallow because its focal lens length is relatively long. When the aperture opens due to lower brightness, the depth of focus becomes extremely shallow. Accordingly, despite the need for highly accurate distance adjustment, it has hardly been possible to adequately perform an automatic focusing operation with the conventional motion picture camera.
In addition to the above problem, the imaging lens of the conventional automatic focusing motion picture camera has been supported and guided by a so-called helicoid arrangement. Lubricating oil used for this helicoid arrangement has a viscosity which is temperature dependent. Under low temperature conditions, therefore, the amount of time required for focusing is prolonged.